1) Field of the Invention
The present invention relates to a positioning apparatus for positioning a movable member such as a gear ratio determining member which determines a gear ratio of an automatic transmission.
2) Description of the Related Art
In an apparatus used for positioning a movable member, such as a positioning table of a machine tool for a work piece, the movable member such as a nut in mesh with an elongated screw is generally actuated by transmitting a turning force generated by a motor to the screw through a gear attached thereto, thereby allowing the nut to travel along an axial direction of the screw.
Recently, demand for high speed and high precision positioning of the nut is increasing. In response to the demand, the use of a motor that can rotate at a high speed such as a brushless motor is increasing.
Positioning stoppers are generally provided at predetermined locations on the screw such as locations close to extreme ends of the screw in the positioning apparatus so as to define a movable range of the movable member. After the nut reaches a target position, the nut is returned to the positioning stopper. In this instance, the nut heavily collides with the positioning stopper unless the rotating speed of the screw, i.e., the rotating speed of the motor, is sufficiently reduced. Accordingly, the stopper and the nut are damaged due to the collision therebetween.
As an example of an approach to prevent damage due to the collision of the positioning stopper and the nut, a limit switch is provided on a contacting face of the positioning stopper against the nut, so that the motor rotation is stopped by using a signal from the limit switch.
Another example for damage prevention is disclosed in Japanese Patent Kokai (Laid Open Publication) No.3-56527, in which a positional detection is made in front of a target stop position, so that a rotating speed of the motor is shifted to a low speed by using a detection signal thereof.
The aforementioned method using the limit switch requires an additional mechanism, i.e., the limit switch, which leads to an increase in overall system cost including a remodeling cost of the system. In addition, the overall reliability of the apparatus is affected by the reliability of the limit switch.
The control system disclosed in Japanese Patent Kokai No.3-56527 requires a position detecting means such as a sensor. In addition, an instructing step is necessary in advance to register a position data for switching over the rotating speed in practical use. Accordingly, this control system becomes complicated and increases an overall system cost that is similar to the control method using the limit switch.